A brake control system for a vehicle is known in the prior art, for example, as disclosed in Japanese Patent Publication H4-121260. According to such a conventional brake control system (also referred to as the first conventional system), an operating speed of a brake pedal is detected, when a vehicle driver operates the brake pedal. And fluid pressure of braking fluid is rapidly increased to its maximum braking pressure, when the detected operating speed of the brake pedal is higher than a predetermined reference value. Namely, in the above first conventional system, whether it is an emergency braking operation or not is determined by the operating speed of the brake pedal. And a brake assisting control to the braking pressure is carried out so that the maximum braking pressure is generated when it is determined that there is the emergency braking operation.
Another brake control system for the vehicle is further known in the prior art, for example, as disclosed in Japanese Patent Publication H11-334557. According to such a conventional brake control system (also referred to as the second conventional system), a relative vehicle distance between a driver's vehicle and a front vehicle (or a front obstacle) is detected by, for example, a laser radar device. A target vehicle deceleration is calculated in order that the driver's vehicle can be stopped before reaching at the front vehicle (or the front obstacle), based on a relative vehicle speed (which corresponds to a rate of change of the relative vehicle distance) or a vehicle speed of the driver's vehicle. And a brake assisting control (a control of pressure increase) to the braking pressure is carried out so that an actual braking pressure is increased to a target braking pressure, which can realize the above target vehicle deceleration.
According to the above first conventional system, the emergency braking operation is simply determined based on the operating speed of the brake pedal. Therefore, the brake assisting control to the braking pressure is always carried out whenever the detected operating speed of the brake pedal is higher than the predetermined reference value, whether or not the vehicle is under the emergency braking operation, such as a situation in which the driver's vehicle is approaching closer to the front vehicle (or obstacle). On the other hand, the brake assisting control to the braking pressure is not carried out, when the vehicle driver can not rapidly operate the brake pedal even in the case of the emergency situation. As above, it is difficult to always carry out the appropriate brake assisting control to the braking pressure, in the case that the brake assisting control to the braking pressure is performed simply depending on the operating speed of the brake pedal.
According to the above second conventional system, it is possible to generate a necessary braking force in the emergency situation, because the brake assisting control to the braking pressure is carried out so that the actual braking pressure is increased to the target braking pressure (the target vehicle deceleration), with which the vehicle can be stopped before the front obstacle. However, if the brake assisting control (the pressure increase control) to the braking pressure was always carried out in order to achieve the target braking pressure so that the vehicle could be stopped before the front obstacle, an extremely high deceleration would be rapidly generated. Accordingly, a decelerating operation having a sense of security to the vehicle driver may not be realized in most cases.